Implantable wireless sensors are useful in assisting diagnosis and treatment of many diseases. Examples of wireless sensor readers are disclosed in U.S. Pat. Nos. 8,154,389, and 8,493,187, each entitled Wireless Sensor Reader, which are incorporated by reference herein. Delivery systems for wireless sensors are disclosed in PCT Patent Application No. PCT/US2011/45583 entitled Pressure Sensor, Centering Anchor, Delivery System and Method, which is also incorporated by reference. In particular, there are many applications where measuring pressure from within a blood vessel deep in a patient's body is clinically important. For example, measuring the pressure in the heart's pulmonary artery is helpful in optimizing treatment of heart failure and pulmonary hypertension. In this type of application, a sensor may need to be implanted 10 to 20 cm beneath the surface of the skin.
Implantable wireless sensors that use radiofrequency (RF) energy for communication and power have been found to be particularly useful in medical applications. However, there are many tradeoffs and design constraints in designing such implantable sensors, such as size, cost and manufacturability.
A key challenge in successful commercialization of these implantable wireless sensors is the design tradeoff between implant size and the “link distance”, which is the physical distance between the implant and the external device communicating with or providing energy to the implant. From a medical standpoint, it is desirable for an implant to be as small as possible to allow catheter based delivery from a small incision, implantation at a desired location, and a low risk of thrombosis following implant. However, from a wireless communication standpoint, the smaller the implant, the shorter the link distance. This distance limitation may be a function of the size of the antenna that can be realized for a given overall implant size. A larger antenna may be able to absorb more RF energy and transmit more RF energy than a smaller antenna. For example, in the case of wireless communication via inductive coupling, a typical implant antenna has the form of a coil of wire. The coil's “axis” is the line that extends normal to the plane of the windings, i.e. the axis is perpendicular to the wire's length. As the area encircled by the coil increases, the amount of magnetic flux that passes through it generally increases and more RF energy is delivered to/received from the implant. This increase in flux through the implant antenna can result in an increase in link distance. Thus to achieve maximum link distance for a given implant size, the implant antenna should be of maximal size.
While antenna size is important, other implant architectures may benefit from maximizing the size of other internal components. An implant containing an energy storage device such as a battery, for example, would enjoy longer battery lifetime with a larger battery. In another example, a drug-eluting implant could hold a larger quantity of the drug. Other examples will be apparent to those skilled in the art. Thus, it may be generally advantageous for an implant to have the largest possible internal volume, while maintaining the smallest possible external dimensions. This objective may be constrained by the implant's need for a strong, biocompatible, and hermetically sealed housing, to protect the internal volume from liquid ingress from the body environment.
Moreover, an optimal implantable sensor may be best designed to function with a specific device or reader device. Wireless transmitter and reader devices, such as the wireless reader of U.S. Pat. No. 9,305,456 and U.S. patent application Ser. No. 13/860,851 entitled “WIRELESS SENSOR READER,” as well as U.S. patent application Ser. No. 14/041,738 entitled “WIRELESS SENSOR READER (SENSOR BANDWIDTH BASED ON AMBIENT CONDITION) which are hereby incorporated by reference herein in their entirety, may require a specific implantable sensor to provide optimal functionality of the reader/sensor system. An optimal implantable sensor for such systems may be configured to transduce pressure into an electrical resonant frequency. The sensor may be a passive sensor with no internal power source, such as a sensor having an LC resonant tank circuit. The sensor may minimize its total size while maximizing coil area, as described in PCT Patent No. PCT/US2012/044998 entitled “IMPLANTABLE SENSOR ENCLOSURE WITH THIN SIDEWALLS,” which is hereby incorporated by reference herein in its entirety. The sensor may have a high RF Quality factor (Q), which is maximized by careful materials selection and device design. The sensor may be immune to temperature changes, including temperature changes during the manufacturing process and in transition between ambient conditions and in vivo. The sensor may have high sensitivity and good electrical isolation between electrical nodes and surrounding body fluids or tissue. The sensor may be highly stable over time, have good mechanical strength, incorporate biocompatible materials, and minimize use of ferritic materials. The sensor may be hermetically sealed to keep blood and other liquids from the body environment away from the internal electronics, possibly for the lifetime of the patient.
For an LC type wireless MEMS sensor, overcoming these challenges requires the design of a small sensor with high RF Quality factor (Q) at low operating frequencies (the human body attenuates wireless data signals, with generally more signal attenuation occurring at higher frequencies above 50 MHz). Additional challenges arise due to regulatory policies and licensed frequency bands for commercial use. With current technology, it is difficult to reliably fabricate an accurate ultra-miniature implantable wireless pressure sensor with high Q factor at low operating frequencies within a tightly controlled operating range.
To improve implantable wireless sensors, it is desirable to optimize various features of the sensor implant to ensure a high resonant quality factor may occur over the life of the implant.